This invention relates to improvements in or relating to remote air detection, in particular to a remote air detector and a method of remote air detection.
The measurement of air velocity and flow direction is important when controlling the motion of an aircraft in flight. Methods currently used employ a pitot tube extending from the aircraft, which enables velocity measurement in terms of the differential pressure between air in the tube and air outside the tube. This method has the disadvantage of only being able to measure velocity close to an airframe of the aircraft where the flow direction of air may be substantially effected by the airframe and is insensitive to low velocities as the differential pressure between air inside and outside the tube is proportional to the velocity of air squared. These disadvantages are particularly acute in helicopters where the forward velocity of air can be very slow or even negative and the flow direction of air, in close proximity to the airframe, can be greatly effected by the downdraft from the engine rotor.
Additionally high performance aircraft in flight, for example supersonic aircraft, operate in unstable aerodynamic configurations and require measurement of airflow direction relative to the aircraft to enable motion to be stabilised.
From FIG. 1, it is known to provide a Doppler anemometer 10 to measure the velocity of flowing air using a laser diode 11 to produce a laser output 12 which is collimated by a collimator lens 13 into a parallel beam 14. The laser diode is orientated to produce a laser output 12 polarised in the direction of propagation of the parallel beam 14 such that the parallel beam 14 is transmitted by a polarising beam-splitter 15 to form a beam 16.
The beam 16 then passes through a quarter-wave plate 17 which converts the linearly polarised light of the beam 16 into a circularly polarised light beam 18. The light beam 18 is expanded by a telescope 19 comprising lenses 20 and 21 and then passes through a window 22 in the aircraft. The beam 18 is brought to focus on a focal point 23 at a distance from the window 22 by adjustment of lenses 20, 21.
Air flowing through the focal point 23 contains particles which may be dust volcanic ash or aerosols in the form of microscopic water droplets. Measurements performed in various parts of the world show that such particles are present everywhere in the atmosphere at a concentration sufficiently to cause a measurable amount of back-scatter from the light beam 18. Hence, a small proportion of light 24 scattered by atmospheric particles close to the focal point 23 is scattered in a direction which returns through window 22, the lenses 20, 21 of telescope 19 and quarter-wave plate 17 to form a colimated signal beam 25. When the light 24 passes through the quarter-wave plate 17 it is converted from circularly polarised light to linearly polarsed light but having a direction of polarisation perpendicular to the propagation direction of the beam 16.
The signal beam 25 is substantially reflected by polarising beam-splitter 15 and is focussed by lens 26 onto the surface of a photodetector 27.
A small proportion of beam 16 is reflected from a plane face 28 of the quarter-wave plate 17 positioned closest to lens 20 and passes back through plate 17 to form a reference beam 29. The portion of beam 16 which is reflected from the plane face 28 of the plate 17 is converted from linearly polarised light to circularly polarised light and then back to linearly polarised light but with a change in the direction of polarisation such that it is perpendicular to the direction of propagation of the reference beam 29. The reference beam 29 is reflected by beam-splitter 15 and is focussed by lens 26 onto the photodetector 27.
The orientation of the quarter-wave plate 17 is adjusted such that the reference beam 29 is accurately parallel and collinear with the signal beam 25. The reference beam 29 and the signal beam 25 form an interference pattern on the surface of the photodetector 27 and, when the plate 17 is properly adjusted, the spacing of interference fringes formed by the beams 25, 29 is substantially larger than the diameter of either beam 25, 29 so that the photodetector 27 receives a light intensity modulated at the difference frequency between the beams 25, 29 which is the Doppler frequency corresponding to the motion of particles at the focal point 23.
The photodetector 27 produces an output current 30 which is passed to a signal analyser 31 which can consist of a fast Fourier transform analyser or a pulse-pair processor that identifies the Doppler frequency which provides an indication of the corresponding air velocity at focal point 23. The air velocity may be displayed on a display 32.
However, the Doppler anemometer described with reference to FIG. 1 is unable to discriminate between positive and negative directions of air flow, which is a particular problem in helicopters able to fly backwards as well as forwards.